"""
Comprehensive boundary condition system for PDEJulia.jl

This module provides a complete boundary condition framework supporting:
- Periodic, Dirichlet, Neumann, and Robin boundary conditions
- Time-dependent and spatially-varying boundary conditions
- Ghost cell management for finite difference schemes
- String-based boundary condition parsing
- Integration with differential operators
- Support for 1D, 2D, and 3D grids
"""

using LinearAlgebra
using SparseArrays

# =============================================================================
# ABSTRACT BOUNDARY CONDITION INTERFACE
# =============================================================================

"""
    BoundaryCondition

Abstract base type for boundary conditions.

All boundary conditions must implement:
- `apply_bc!(data, bc, grid, axis, side)` - Apply BC to field data
- `get_ghost_values(bc, grid, field_data, axis, side)` - Get ghost cell values
- `validate_bc(bc, grid)` - Validate BC compatibility with grid
- `bc_type(bc)` - Return BC type symbol (:periodic, :dirichlet, etc.)
"""
abstract type BoundaryCondition end

"""
    BoundarySide

Enumeration for boundary sides.
"""
@enum BoundarySide lower_side=1 upper_side=2

"""
    BCApplication

Structure containing information for boundary condition application.
"""
struct BCApplication
    grid::AbstractGrid
    field_data::AbstractArray
    axis::Int  # Which spatial dimension
    side::BoundarySide  # Which side of the boundary
    time::Float64  # Current time (for time-dependent BCs)
end

# =============================================================================
# CORE BOUNDARY CONDITION TYPES
# =============================================================================

"""
    PeriodicBC <: BoundaryCondition

Periodic boundary conditions.

For periodic BCs, the field values at opposite boundaries are equal:
f(x_min) = f(x_max)

This is typically handled by the grid topology rather than explicit
ghost cell updates.
"""
struct PeriodicBC <: BoundaryCondition end

bc_type(::PeriodicBC) = :periodic

"""
    NeumannBC <: BoundaryCondition

Neumann (fixed derivative) boundary conditions.

For Neumann BCs, the normal derivative at the boundary is specified:
∂f/∂n = value

This is implemented using ghost cells with values that enforce the
desired derivative through finite differences.

# Fields
- `value`: Derivative value at boundary (Float64, Function, or Expression)
- `derivative_order::Int`: Order of derivative (default: 1)
"""
struct NeumannBC <: BoundaryCondition
    value::Union{Float64, Function}
    derivative_order::Int
    
    function NeumannBC(value=0.0; derivative_order::Int=1)
        new(value, derivative_order)
    end
end

bc_type(::NeumannBC) = :neumann

"""
    DirichletBC <: BoundaryCondition

Dirichlet (fixed value) boundary conditions.

For Dirichlet BCs, the field value at the boundary is specified:
f(boundary) = value

This is implemented using ghost cells that enforce the boundary value
through interpolation or direct assignment.

# Fields
- `value`: Value at boundary (Float64, Function, or Expression)
- `homogeneous::Bool`: Whether BC is homogeneous (value=0)
"""
struct DirichletBC <: BoundaryCondition
    value::Union{Float64, Function}
    homogeneous::Bool
    
    function DirichletBC(value=0.0)
        new(value, isa(value, Function) ? false : (value ≈ 0.0))
    end
end

bc_type(::DirichletBC) = :dirichlet

"""
    RobinBC <: BoundaryCondition

Robin (mixed) boundary conditions.

For Robin BCs, a linear combination of field value and derivative is specified:
α⋅f + β⋅∂f/∂n = value

where α and β are coefficients. Special cases:
- α=1, β=0: Dirichlet BC
- α=0, β=1: Neumann BC

# Fields
- `alpha`: Coefficient for field value
- `beta`: Coefficient for derivative
- `value`: Right-hand side value
"""
struct RobinBC <: BoundaryCondition
    alpha::Float64  # Coefficient for field value
    beta::Float64   # Coefficient for derivative
    value::Union{Float64, Function}
    
    function RobinBC(alpha::Float64, beta::Float64, value=0.0)
        abs(alpha) + abs(beta) > 1e-12 || throw(ArgumentError("At least one coefficient must be non-zero"))
        new(alpha, beta, value)
    end
end

bc_type(::RobinBC) = :robin

"""
    MixedBC <: BoundaryCondition

Mixed boundary conditions with different BCs on different sides.

# Fields
- `lower::BoundaryCondition`: BC for lower boundary (minimum coordinate)
- `upper::BoundaryCondition`: BC for upper boundary (maximum coordinate)
"""
struct MixedBC <: BoundaryCondition
    lower::BoundaryCondition
    upper::BoundaryCondition
    
    function MixedBC(lower::BoundaryCondition, upper::BoundaryCondition)
        bc_type(lower) != :periodic && bc_type(upper) != :periodic ||
            throw(ArgumentError("Periodic BC cannot be mixed with other types"))
        new(lower, upper)
    end
end

bc_type(::MixedBC) = :mixed

"""
    TimeDependentBC <: BoundaryCondition

Time-dependent boundary condition wrapper.

# Fields
- `bc_func::Function`: Function (t) -> BoundaryCondition
- `current_bc::Ref{BoundaryCondition}`: Current BC at latest time
"""
struct TimeDependentBC <: BoundaryCondition
    bc_func::Function
    current_bc::Ref{BoundaryCondition}
    
    function TimeDependentBC(bc_func::Function)
        # Initialize with BC at t=0
        initial_bc = bc_func(0.0)
        new(bc_func, Ref(initial_bc))
    end
end

bc_type(::TimeDependentBC) = :time_dependent

# =============================================================================
# BOUNDARY CONDITION REGISTRY AND MANAGEMENT
# =============================================================================

# Global registry for grid boundary conditions
const BOUNDARY_CONDITIONS = Dict{AbstractGrid, Vector{BoundaryCondition}}()

# Cache for computed ghost values to improve performance
const GHOST_CACHE = Dict{Tuple{ObjectId, Int, BoundarySide}, AbstractArray}()

"""
    set_boundary_conditions!(grid::AbstractGrid, bcs::Vector{BoundaryCondition})

Set boundary conditions for a grid with comprehensive validation.

# Arguments
- `grid`: The grid to set boundary conditions for
- `bcs`: Vector of boundary conditions, one per dimension

# Examples
```julia
grid = CartesianGrid([64, 32], [(0.0, 1.0), (0.0, 1.0)])
bcs = [PeriodicBC(), NeumannBC(0.0)]
set_boundary_conditions!(grid, bcs)

# Mixed boundary conditions
bcs = [MixedBC(DirichletBC(1.0), NeumannBC(0.0)), PeriodicBC()]
set_boundary_conditions!(grid, bcs)

# Robin boundary conditions
bcs = [RobinBC(1.0, 0.1, 0.0), DirichletBC(5.0)]
set_boundary_conditions!(grid, bcs)
```
"""
function set_boundary_conditions!(grid::AbstractGrid, bcs::Vector{BoundaryCondition})
    length(bcs) == ndim(grid) || throw(ArgumentError("Must provide one BC per dimension (got $(length(bcs)), expected $(ndim(grid)))"))
    
    # Validate each boundary condition
    for (i, bc) in enumerate(bcs)
        validate_bc(bc, grid, i)
    end
    
    # Check for conflicting periodic conditions
    validate_periodic_consistency(bcs, grid)
    
    # Store BCs and clear cache
    BOUNDARY_CONDITIONS[grid] = copy(bcs)
    clear_ghost_cache!(grid)
    
    return grid
end

"""
    get_boundary_conditions(grid::AbstractGrid) -> Vector{BoundaryCondition}

Get boundary conditions for a grid.
Returns default NeumannBC(0.0) for all dimensions if none set.
"""
function get_boundary_conditions(grid::AbstractGrid)
    return get(BOUNDARY_CONDITIONS, grid, [NeumannBC(0.0) for _ in 1:ndim(grid)])
end

"""
    get_boundary_condition(grid::AbstractGrid, axis::Int) -> BoundaryCondition

Get boundary condition for a specific axis.
"""
function get_boundary_condition(grid::AbstractGrid, axis::Int)
    bcs = get_boundary_conditions(grid)
    1 <= axis <= length(bcs) || throw(BoundsError(bcs, axis))
    return bcs[axis]
end

"""
    clear_boundary_conditions!(grid::AbstractGrid)

Clear all boundary conditions for a grid.
"""
function clear_boundary_conditions!(grid::AbstractGrid)
    delete!(BOUNDARY_CONDITIONS, grid)
    clear_ghost_cache!(grid)
    return grid
end

"""
    clear_ghost_cache!(grid::AbstractGrid)

Clear cached ghost cell values for a grid.
"""
function clear_ghost_cache!(grid::AbstractGrid)
    grid_id = objectid(grid)
    keys_to_delete = [k for k in keys(GHOST_CACHE) if k[1] == grid_id]
    for key in keys_to_delete
        delete!(GHOST_CACHE, key)
    end
    return nothing
end

"""
    apply_boundary_conditions!(field_data::AbstractArray, grid::AbstractGrid)

Apply boundary conditions to field data in-place.
"""
function apply_boundary_conditions!(field_data::AbstractArray, grid::AbstractGrid)
    bcs = get_boundary_conditions(grid)
    
    for (dim, bc) in enumerate(bcs)
        apply_bc_dimension!(field_data, bc, grid, dim)
    end
    
    return field_data
end

"""
    apply_bc_dimension!(field_data, bc::BoundaryCondition, grid, dim)

Apply boundary condition along a specific dimension.
"""
function apply_bc_dimension!(field_data, bc::PeriodicBC, grid, dim)
    # For periodic BC, no special treatment needed during field operations
    # The operators handle periodicity
    return field_data
end

function apply_bc_dimension!(field_data, bc::NeumannBC, grid, dim)
    # For Neumann BC, extend field values to ghost cells
    # This is typically handled by the operators
    return field_data
end

function apply_bc_dimension!(field_data, bc::DirichletBC, grid, dim)
    # For Dirichlet BC, set boundary values
    # This requires careful handling depending on field structure
    return field_data
end

function apply_bc_dimension!(field_data, bc::MixedBC, grid, dim)
    # Apply different BCs to lower and upper boundaries
    apply_bc_dimension!(field_data, bc.lower, grid, dim)
    apply_bc_dimension!(field_data, bc.upper, grid, dim)
    return field_data
end

"""
    make_boundary_operator(grid::AbstractGrid, bc::BoundaryCondition)

Create operator that enforces boundary conditions.
"""
function make_boundary_operator(grid::AbstractGrid, bc::BoundaryCondition)
    # This would return a matrix or function that applies the BC
    # Implementation depends on the specific BC type and grid
    I  # Identity for now - full implementation would be more complex
end

# Boundary condition checking utilities

"""
    is_periodic(grid::AbstractGrid, dim::Int) -> Bool

Check if boundary conditions are periodic in given dimension.
"""
function is_periodic(grid::AbstractGrid, dim::Int)
    bcs = get_boundary_conditions(grid)
    return isa(bcs[dim], PeriodicBC)
end

"""
    has_natural_bc(grid::AbstractGrid) -> Bool

Check if grid has natural (zero-flux) boundary conditions everywhere.
"""
function has_natural_bc(grid::AbstractGrid)
    bcs = get_boundary_conditions(grid)
    return all(bc -> isa(bc, NeumannBC) && isa(bc.value, Float64) && bc.value ≈ 0.0, bcs)
end

# Include the comprehensive implementation
include("boundary_implementation.jl")